Virtually any fish can be caught on the fly. The main species targeted by the fly angler in the past were trout, salmon, grayling and to a lessor extent char. Nowadays anglers are constantly searching for new horizons and all manner of coarse fish and sea fish are now sought after with relish. Pike, for example, readily take the fly as do mackerel. Both are excellent sport when hooked on fly tackle.
Casting the fly into the sea is really quite fashionable at the moment. Bone fish in Florida and Easter Island have become prime targets, due mainly to their fantastic fighting ability. However, as this is a stillwater guide, we shall restrict ourselves to such and consider the two main stocked fish in reservoirs today.
They are the Rainbow trout (Onchorhynchus mykiss) and the Brown Trout (Salmo trutta). Only the brown trout is indigenous to the UK.
Rainbow trout are so named due to their colouring, ranging from olive green on the back, to violet on the flank, finishing with a pale underside. They are marked with small dark spots, beginning somewhere just beyond the cheeks, continuing all the way to the end of the tail. Generally speaking, reservoir rainbows once naturalised after stocking, take on a silvery sheen.
Browns on the other hand have undersides ranging from a pale white through to a buttery yellow, with silver or brown flanks and a darker back. The brown has larger spots than its rainbow cousin, none on the tail but some markings on the cheeks.
The virtues of each fish are thoroughly argued about by anglers. There does exist perceived behavioural differences between the two species. Brown trout become somewhat dour in the hotter summer weather. They tend to fight less dramatically but with perhaps more doggedness. Rainbows have a better tolerance to the warmer water temperature and their fighting often takes on an aerial aspect. Whether browns are harder to catch is a matter of opinion, however, I personally believe that browns became selective feeders more often and once in such a mood, are much harder to introduce to take a lure. Also, rainbows appear to be more partial to bright colours than browns, or certainly that is the general opinion. My experience leads me to be not so sure.
Trout have a good sense of vision (in colour), taste, smell and hearing. In addition they have a lateral line which is sensitive to underwater vibrations. They can't talk.
Trout are not particularly intelligent creatures. Most of their behaviour is instinctive. That does not mean that they are any easier to catch, frequently the reverse. Education makes one trout harder to catch than the other. It is said that once a trout has been caught and released six times it becomes virtually uncatchable. Although this is not true in every case, it is true to say that they become much, much harder to catch.
Even trout in waters where catch and release is not allowed can become educated after being hooked and lost, or pricked and missed, or just shown plenty of flies. Consider the trout in such a reservoir in the UK, chances are, every day he gets to see plenty of new flies. Once that trout has been in the water for half a season or more his capture becomes really quite a challenge.